1. Field of the Invention
The present invention relates to a thermal recording apparatus, and, more particularly, to a thermal recording apparatus such as a thermal printer or a heat transfer printer which has heating elements with variably controlled energization.
2. Description of the Prior Art
In a facsimile apparatus, for instance, the energy supplied to the recording elements, such as the heating elements in a thermal recording apparatus, is controlled in accordance with a reception mode (G3 mode or G2 mode), a standard or fine mode, or a mode for each document.
With such conventional structure, the time required for encoding and decoding one line can vary depending on the image pattern and, when variations occur in the recording cycle for each line, irregularities may occur in the density of the recorded image.
For instance, in a fine recording mode which generally has a longer recording cycle than the standard mode, the amount of energy supplied to the heating elements is set relatively high. However, when lines having complicated image patterns and simple image patterns are present on a single page, the processing time required for decoding, such as determining the run length, will be short for lines in which the pattern is simple and, since such line will thus be recorded very shortly after the preceding line, the heating elements may accumulate heat, so that the recorded image is to dark.
Conversely, for a line which has a complicated pattern, and a long recording cycle, heat will not be accumulated from recording of the preceding line, so that the recorded image may become lighter, and thus irregularities in density may occur in one page of the recorded image.
Consequently, attempts have been made to control heat generation in this type of apparatus by taking into account the heat accumulated by the recording head and thus maintain high-quality recording. One method uses a thermistor or other temperature detecting device in the recording head, and the amount of heat generated is controlled by controlling the drive of the heating elements in accordance with the output of the detecting device. In another method, heat generation is controlled in accordance with the number of black dots (recording dots) in the recording data. Nevertheless, these conventional methods have not resulted in a significant improvement in recording quality, and irregularities in the density of the recorded image still may occur.
For instance, in a thermal printer for a facsimile apparatus, image data received by a main control unit is decoded from a mode such as MH or MR code and the decoded data is subsequently received and recorded by the printer, so that the recording time interval per line may not be constant. Consequently, the amount of heat generated by the head per line may vary, thereby resulting in irregularities in the density of the recorded image, because the thermistor cannot provide a sufficiently accurate temperature detection.
In addition, with a conventional system which controls the amount of heat generated by controlling the drive of the heating elements on the basis of the number of recording dots in a line of the recording data, irregularities in the density of the recorded image may still occur, particularly in a line-type head, if there is an uneven distribution of recording dots within a line, since the amount of heat generated is controlled on the basis of the number of recording dots in the entire line.